The tension in relations between Russia and the West is at a more dangerous level today than at any time since the first Reagan administration (1980-84).

So it is worth recalling a period (largely overlapping with the second Reagan administration) when Russia was becoming a freer country by the month.

It was a death in Moscow 30 years ago today - 10 March 1985 - that opened the door to domestic reform and to dramatic change in the political map of Europe.

Konsantin Chernenko, the 73-year-old conservative Communist leader of the Soviet Union, died, and the number two man in the Soviet hierarchy, Mikhail Gorbachev, promptly convened a meeting of the Politburo, the Communist Party's ruling body.

It is salutary to remember just how much changed, mainly for the better, in the period when Gorbachev was the Soviet Union's last ruler

Some of those present would have liked to stop the further rise of Mr Gorbachev, who at 54 was the youngest member of the top leadership team, but they did not have a plausible alternative candidate.

By the afternoon of 11 March, Mr Gorbachev had been unanimously elected by the Central Committee as general secretary of the Communist Party and thus leader of the world's second superpower.

A combination of the difficulties the Soviet Union faced and the authority of the general secretaryship enabled Mr Gorbachev to launch his perestroika (reconstruction), which became a synonym for increasingly radical political innovation.

The new Soviet leader was already more of a reformer than his Politburo colleagues realised. In power, his policies became bolder and more far-reaching. He had an unusually open mind for a Communist politician.

The more, however, Mr Gorbachev reformed the Soviet system, the more he undercut the traditional authority of the party leader - his own powerbase - while the new tolerance brought countless long-suppressed problems, including nationalist discontent, to the surface of political life.

So much so that by 1990 the continuing existence of the Soviet Union was in jeopardy.

In December 1991, Mr Gorbachev's efforts to recreate the union as a voluntary federation ended in failure. The country dissolved into 15 successor states.

It is salutary to remember just how much changed, mainly for the better, in the period when Mr Gorbachev was the Soviet Union's last ruler - the principal architect of the transformation and its crucial facilitator.

Here are some of the internal changes, and, if anything, the even more momentous, international changes:

Internal changes:

A new policy of glasnost (transparency) quite rapidly developed into freedom of speech

Hitherto banned books that challenged not only official history but also the legitimacy of Communist rule were published in large editions

Dissidents were released from prison and past distortions of justice were investigated

Persecution of the Churches ceased and gave way to religious tolerance

Contested elections in which votes were counted honestly were introduced in 1989

The official ideology, Marxism-Leninism, was increasingly discarded and replaced by ideological pluralism and free intellectual inquiry

Freedom of communication (including an end to jamming of foreign broadcasts) and liberty to travel to Western countries were introduced

So-called "democratic centralism" within the Communist Party - a euphemism for a hierarchical, strictly-disciplined party, intolerant of dissent - was quietly abandoned. The party of almost 20 million members became an openly argumentative body with the clash of ideas out in the open. Communist Party members stood against one another on radically different political platforms in the elections of 1989, 1990 and 1991

Constitutional change in early 1990 legalised the creation of oppositional political parties and legitimised the already existing new political pluralism

International changes:

From the outset, Mr Gorbachev made clear within the leadership that Soviet troops must leave Afghanistan, which they had invaded in 1979. In February 1989, the last Soviet soldier departed

The Soviet Union's last leader established good personal relations with Western political leaders. Annual summit meetings with American presidents had by 1987 produced substantive results, including reductions in the size of the military superpowers' nuclear arsenals and the removal of their short-range and medium-range nuclear missiles from Europe

In major speeches in 1988 - Moscow in June and the UN in New York in December 1988 - Mr Gorbachev renounced the "Brezhnev doctrine" of limited sovereignty for the countries of Eastern Europe. He declared that in all countries the people themselves were entitled to decide what kind of political and economic system they wished to live in

In, arguably, the most important change of all, the peoples of Eastern Europe in 1989 took Mr Gorbachev at his word. They removed their Communist leaders from power (peacefully, except in Romania) and not a shot was fired by a Soviet soldier as the countries of Central and Eastern Europe became independent and non-Communist

It had long been assumed in Western capitals that, for the Soviet Union, the continued division of Germany was non-negotiable. Yet that country was reunited in 1990. The misleadingly named German Democratic Republic (Communist East Germany) ceased to exist as a political entity and became part of the enlarged Federal Republic of Germany

By 1991 relations between Russian leaders (especially with Mr Gorbachev as Soviet President but also with Boris Yeltsin as President of the Russian republic) and their Western counterparts were warm and trusting.

The failure of East and West to build on the new foundations is a tragedy of major proportions.

Archie Brown is emeritus professor of politics, University of Oxford, and the author of The Gorbachev Factor (1996) and Seven Years that Changed the World: Perestroika in Perspective (2007).